Cellular networks are divided into many small geographic areas, called cells or sites. Each cell is adjacent to one or more cells. Collectively, the cells provide cellular service to a large geographic area. Each cell is typically served by one or more corresponding base stations. Within each cell, the one or more corresponding base stations serve one or more mobile terminals (or mobile stations) situated within the cell. As further detailed below, other equipment, such as a relay, which aids in base station—mobile terminal communications may also serve a cell.
Signals propagating through a given cell may include transmissions from equipment within the cell (e.g. mobile terminals and base stations within the cell), and may also include signals transmitted from adjacent cells. Thus, in certain instances, a mobile terminal may receive relatively strong signals from multiple transmitters. For example, while situated near the border of one cell and an adjacent cell, a mobile terminal may receive signals transmitted from both the base station serving the cell within which the mobile terminal is situated, and from the base station serving the adjacent cell. Moreover, a mobile terminal may receive signals from multiple sources within a given cell, such as relays and other base stations. Signals from these various sources may interfere, for example, by constructive superposition of signals from the various sites, with the signal expected or desired to be received by the mobile terminal.
Consequently, it would be advantageous if the various signals received by a mobile terminal could be combined in such a way as to transform what would otherwise be interference into a useful signal from the perspective of the mobile terminal. Various techniques of doing so have been proposed and generally include: cooperation between base stations and relays, between relays and base stations with distributed antennas, within the same cell; and between base stations of two or more different cells.
Open loop cooperation between the cooperating equipment typically involves transmit diversity schemes/techniques and spatial multiplexing schemes. Transmit diversity schemes included band switching transmit diversity wherein different sub-bands were allocated to a particular mobile terminal in the cooperating sites; phase delay diversity (PDD)/short cyclic delay diversity (CDD) wherein phase delay or cyclic delay was applied to the signal to generate spatial diversity through forward error correction; and space-time-frequency transmit diversity wherein different cooperating sites used the same resource using space-tone codes. In the spatial multiplexing scheme, different cooperating sites transmitted independent data streams to the receiver. A drawback of this known open loop cooperation technique was that it did not exploit channel state information (CSI) feedback to the cooperating site. Consequently, this technique was more useful for medium and high speed users and less useful for low speed users where channel state information could be exploited to provide better quality of service.
Another technique, interference alignment, employs multi-site multi-user MIMO (MU-MIMO) techniques wherein different MIMO cooperating sites transmitted sets of independent data to different users using the same shared resource. The cooperating sites aligned their induced interferences at all nodes. Drawbacks of this technique included that it could only be applied to high geometry users, required pairing up of two or more users serviced by the same base station and required knowledge by the transmitting site (e.g. base station) channel conditions. Since channel condition data or information from which channel conditions could be calculated by the transmitting site had to be transmitted from the mobile terminals, this technique resulted in higher feedback overhead. Consequently, this technique was largely applied to fixed or low speed users.
A need therefore exists for improved techniques for multi-transmitting site cooperation.